It is proposed to determine the structure of deoxy sickle cell hemoglobin by means of X-ray diffraction from single crystals. Splendid X-ray quality crystals of deoxy sickle cell hemoglobin have been grown. It is proposed to determine how to position and orient the deoxy horse hemoglobin molecule in the asymmetric unit of the deoxy sickle cell crystal lattice in order to best account for the diffraction pattern. The phases from this model will be combined with the measured amplitudes from deoxy Hb S crystals to give Fourier coefficients with which a map of the electron density will be calculated.